[dev.simd] simd: refactor intrinsics definition

This breaks a large template full of defined sections into
a few constant strings, some plain templates, and a map of
templates.

Change-Id: I2051fa0fc71e8bda05a82cffb0ca5b97fc55b5f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/778863
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>